Optical codes typically comprise a pattern of dark elements and light spaces. There are various types of optical codes, including 1-D codes (such as UPC and EAN/JAN barcodes) and 2-D codes (such as PDF-417 and Maxicode). For convenience, some embodiments are described herein with reference to 1-D barcodes. However, the embodiments may also be useful for other optical codes and symbols, and nothing herein should be construed as limiting the embodiments to barcodes.
A conventional barcode label comprises a series of parallel dark bars of varying widths with intervening light spaces, also of varying widths. The information encoded in the barcode is represented by the specific sequence of bar and space widths, the precise nature of this representation depending on which particular barcode symbology is in use. Typical methods for reading barcodes comprise generation of an electrical signal wherein a signal voltage alternates between two preset voltage levels, one representing a dark bar and the other representing a light space. The temporal widths of these alternating pulses of high and low voltage levels correspond to the spatial widths of the bars and spaces. It is this temporal sequence of alternating voltage pulses of varying widths that is presented to an electronic decoding apparatus for decoding.
One common type of data reader is flying spot scanner, in which a beam of light is moved (i.e. scanned) across the optical code while a photodetector monitors the reflected or backscattered light. For example, the photodetector may generate a high voltage when a large amount of light scattered from the optical code impinges on the detector, as from a light area of the optical code, and likewise may produce a low voltage when a small amount of light scattered from the optical code impinges on the photodetector, as from a dark area or bar of the optical code. The beam of light in spot scanners is typically a laser, but may comprise a coherent light source (such as a laser or laser diode) or non-coherent light source (such as light emitting diode). A laser may offer advantages of higher intensity illumination, which may allow optical codes to be read over a larger range of distances from the scanner (large depth of field) and under a wider range of background illumination conditions.
The reading spot generated by the illumination source of a scanner may also be manually moved across a barcode—this type of reader being typically referred to as a wand. Alternately, the reading spot may be automatically moved or scanned across the optical code in a controlled pattern. A scan engine may comprise a rotating mirror facet wheel, an oscillating mirror, or other suitable means for repetitively moving the illumination beam. The path of the reading spot created on an object by the moving illumination beam is referred to as a scan line. Typically, an individual scan line must extend across the optical code for the optical code to be successfully read, unless specialized piecing software (known as stitching) or electronics are utilized. In addition to the scan engine, a scanner may also employ a set of scan pattern generating optics to produce a multiplicity of scan lines in various directions from the scanner and at varying orientations, thereby allowing optical codes to be read over a large angular field of view and over a wide range of orientations (i.e., a multidirectional or omnidirectional scan pattern). The scan pattern generating optics typically comprise a set of mirrors aligned at varying angles, each of the mirrors intercepting the illumination beam during a portion of its motion and projecting the beam into the region in front of the scanner. Each mirror or mirror set, in conjunction with the scanning mechanism, produces a scan line at a particular position and at a particular orientation.
One type of data reader is known as a presentation scanner. An example of a presentation scanner with a handheld capability is described in U.S. Pat. No. 6,575,368, which is incorporated herein by reference. Presentation scanners are useful for scanning in environments that have limited counter space, such as convenience stores. As compared to sweep scanners, which may rely in part on movement of an optical code through a scan volume, presentation scanners are designed to read an optical code of an item held relatively stationary in the scan volume. Presentation scanners are also typically easier to set up than sweep scanners of the type commonly used in supermarkets, which are typically installed in the checkout counter. Certain scanners described in the '368 patent allow both fixed and handheld operation, and may provide distinct optical scan patterns or methods for each mode of operation. In the handheld mode of operation, the scanner is moved to the optical code for reading and a single scan line is selectively generated and/or decoded. In the fixed mode of operation, a relatively wide angular field of view is preferred so that an optical code can be read across a large fraction of the surface of a barcoded object. Since objects are often passed through or presented in the scan volume in random orientations, a multidirectional or omnidirectional scan pattern is desirable for efficiently reading the optical code. In addition, a high scan rate is desirable to allow successful reading of optical codes that are quickly passed through the scan volume.
Another type of data reader is an imaging reader, such as a CCD (charge coupled device) reader. Imaging readers can be configured to read both 1-D and 2-D optical codes, as well as other types of optical codes and symbols. When an imaging reader is used to read an optical code, an image of the optical code or portion thereof is focused onto a detector array. An imaging reader may utilize a light source to illuminate the item being scanned, to provide the required signal response in the imaging device. Imaging technology has heretofore primarily been used for machine vision systems, hand-held barcode readers, and document scanners. For the purposes of this description, the term “scanner” may refer to data readers of either the spot scanner type or the imaging reader type, or both.
With the advent of low-cost imagers and high-speed processors, it is now possible to economically implement imaging readers in a presentation scanner. However, the present inventors have recognized that the high-intensity illumination often required for good performance of an imaging reader can be annoying to the user, especially if employed in a presentation scanner. Implementing an imaging reader in a presentation scanner may cause the high-intensity illumination to be emitted in the general direction of a user (e.g., a clerk) or a bystander (e.g., a customer). This high-intensity illumination can be uncomfortable to look at for an extended period of time. Thus, the present inventors have identified a need for reducing the amount of time that a user or bystander is exposed to high-intensity illumination without substantially affecting data reader performance.